This invention relates to a multi-cycle power generator. More specifically, the invention relates to a power generator having a heat pump cycle and at least a hot power generating cycle, the cycles being configured relative to one another to transfer heat there between two or more times in a single cycle thereby to recover energy through the reuse of unused heat energy that is typically discarded in conventional thermodynamic cycles. The invention may further include a cool power generating cycle in combination with the heat pump and hot power generating cycles.
Systems incorporating energy recovery for the purposes of maximising efficiency are well known, particularly in the air conditioning industry where systems to heat and/or cool building interiors generally consume large amounts of power.
Energy recovery in these systems can take many forms. For example, patent document U.S. Pat. No. 5,136,854 discloses a refrigeration cycle with a compressor thereof shaft mounted to a turbine of a secondary hot power cycle. The turbine is powered by the secondary hot power cycle, and in turn transmits power to the compressor of the refrigeration cycle through the shaft. In this manner, the power normally required to drive the compressor is reduced, thereby making the system more efficient.
Another example is the invention described in international patent application no. PCT/US2007/064506 (published as WO 2008/115236). This patent document discloses the principle of energy recovery by combining a power cycle with a heat pump cycle, the latter of which already being regarded as a very efficient means of pumping heat (typical coefficient of performance of between 3 and 8). In this system, heat is transferred from the heat pump cycle to the power cycle via a common heat exchanger.
Yet another example is a solar thermal energy system developed by Tas Energy (www.tas.com), which combines a steam Rankin cycle with an organic Rankin cycle through a common heat exchanger. In this manner, the heat that would typically be discarded from the Rankin cycle is transferred to the organic Rankin cycle, having a working fluid with a boiling point lower than the working fluid of the Rankin cycle.
Although the energy recovery methods made use of in the aforementioned prior art systems already work to better the efficiency of such systems, they do not make use of multiple (i.e. at least double) heat transfer between cycles, which allows heat to be recycled through the heat pump to increase energy recovery efficiency. It is envisaged that by further combining hot and cool power cycles with a heat pump cycle, the further increase in efficiency will be significant as compared to known systems.
It is common knowledge that that energy cannot be created or destroyed, only converted—for example, converting heat energy into kinetic energy. It is also common knowledge that a heat pump is capable of not only moving heat from one location to another, but also capable of itself transferring heat energy to the working fluid by the work it performs thereon.
Heat pumps typically have a coefficient of performance (COP) of above 1 in respect of moving heat. However, the COP of heat pumps is respect of heating the working fluid is generally less than 1, which is not as good as a heating element. As such, heat pumps are very efficient at moving heat, but not as efficient at heating.
What follows is an explanation of the advantages of using a heat pump, in accordance with the present invention, which typically has a COP of 3 to 8 when it comes to heating objects. This means that 1 unit of energy is required to move 3 to 8 units of energy up a gradient. The efficiency of the system is determined by many factors including the size of the gradient of energy transfer, meaning that heat can be extracted out of the atmosphere or other heat sources, and “concentrated” to a useful level for, amongst other things, boiling fluids.
The steam pressure generated, and/or temperature thereof, can then be used to generate electricity, which can consequently be used to at least partially drive the heat pump, used to power other devices and/or be stored. The energy utilization of the steam is not always that efficient, particularly at lower heat and pressures.
To overcome this, it is proposed that the unused energy is recycled or reused, with the goal being to convert the energy into electricity of an amount similar to the amount of energy absorbed at the heat source. In other words, each time the energy is recycled, more energy is presented to the turbine until the above goal is achieved, thereby countering the inefficiencies of most commercially available turbines.
A further proposal is to increase the efficiency of the system by using the temperature drop generated on the cold side of the heat pump to generate electricity using a different working fluid.
The increased efficiency comes from the fact that almost no added energy needs to be added to the system, for this extra generation of the electricity. This gives a second opportunity to extract energy from the heat source.
A further way of improving the efficiency of the system is by embedding the system in thermal insulation, because much of the energy can be lost through heat loss.
The theoretical principle of it is envisaged that the multi-cycle power generator will operate is set out below. The formulae to run a heat pump is as follows:
  COP  =      Q    W  
where                Q is the heat supplied to or removed from the reservoir; and        W is the work consumed by the heat pump.        
The COP for heating and cooling are thus different. For cooling, the COP is the ratio between the heat removed from the cold reservoir to input work. For heating, the COP is the ratio between the heat removed from the cold reservoir plus the heat added to the hot reservoir to the input work:
            COP      heating        =                                                  Q            H                                    W            =                                                              Q              C                                            +          W                W                        COP      cooling        =                                    Q          C                            W      
where                QC is the heat removed from the cold reservoir; and        QH is the heat supplied to the hot reservoir.        
According to the first law of thermodynamics, in a reversible system we can show that Qhot=Qcold+W and W=Qhot−Qcold, where Qhot is the heat transferred to the hot reservoir and Qcold is the heat collected from the cold reservoir.
Therefore, by substituting for W:
      COP    heating    =            Q      hot                      Q        hot            -              Q        cold            
For a heat pump operating at maximum theoretical efficiency (i.e. Carnot efficiency), it can be shown that:
            Q      hot              T      hot        =                              Q          cold                          T          cold                    ⁢                          ⁢      and      ⁢                          ⁢              Q        cold              =                            Q          hot                ⁢                  T          cold                            T        hot            
where Thot and Tcold are the temperatures of the hot and cold heat reservoirs respectively. Note that these equations must use an absolute temperature scale, for example, Kelvin or Rankine.
At maximum theoretical efficiency:
      COP    heating    =            T      hot                      T        hot            -              T        cold            
which is equal to the reciprocal of the ideal efficiency for a heat engine, because a heat pump is a heat engine operating in reverse. Similarly:
      COP    cooling    =                    Q        cold                              Q          hot                -                  Q          cold                      =                  T        cold                              T          hot                -                  T          cold                    
The COP of the hot side is greater than the COP of the cold side. This is due to the fact that the heat rejected to the hot sink is equivalent to the amount of heat extracted from the cold source plus the heat generated from the heat pump engine, whereas the cold source looses heat without the benefit of the added energy of the heat pump. In other words, the cold source gets less cold than the heat sink gets hot.
COPheating applies to heat pumps and COPcooling applies to air conditioners or refrigerators. For heat engines, values for actual systems will always be less than these theoretical maximums.
As the formula shows, the COP of a heat pump system can be improved by reducing the temperature gap Thot minus Tcold at which the system works. For a heating system this would mean two things:                1. reducing the output temperature to around 30° C. (86° F.), which would require piped floor, wall or ceiling heating, or alternatively, oversized water to air heaters; and        2. increasing the input temperature (e.g. by using an oversized ground source or by access to a solar-assisted thermal bank.        
The heat pump itself can be improved by increasing the size of the internal heat exchangers relative to the power of the compressor, and to reduce the system's internal temperature gap over the compressor.
It is therefore an object of the present invention to provide a power generator comprising of a hot power cycle, a cool power cycle and a heat pump cycle, working on the principles set out above, which power generator may be a stand alone system or part of another system.